The only reason a prospective client says, "we're going to do it ourselves" is because they don't see the value in outsourcing it.
When the salesperson makes it their job to demonstrate value throughout the meeting, this objection will not occur. However.....on the odd occasion that you don't take the opportunity to demonstrate value, try asking
"What part of (the service you offer) do you enjoy the most?"
Once the prospective client has shared what s/he enjoys the most about (the service you offer) you can demonstrate how your knowledge is the missing link.
-Terri